In a bed having a bed-mat containing fine, movable particles, the temperature of air to be fed to suspend the particles in the mat increases due to adiabatic compression by a compressor. For this reason, a temperature sensor is usually included to detect the temperature of the mat and to send an output signal to a temperature controller. The controller operates a cooler fan motor in accordance with the temperature of the mat to maintain the temperature of compressed air to be supplied into the mat at a constant level.
This system, however, has been inconvenient where the ambient temperature abruptly rises or where the cooler or the controller is inoperable because of failure or malfunction. In these cases, a particle temperature can easily exceed a predetermined temperature and present a serious problem for a patient lying on the bed.
For this reason conventional devices have required an additional sensor which is operable at a temperature beyond the predetermined level. This additional sensor controls the operation of the compressor, the fan motor and other devices to prevent the mat temperature from rising further and eventually lowers the temperature to a correct level. This structure, however, also suffers from shortcomings. When the supply of compressed air stops with the operation of the additional sensor, the mat surface holds a particular surface shape corresponding to the patient's body. The surface shape is vulnerable and unstable, however, when the patient on the bed moves and the distribution of the load of the patient is shifted on the mat.
Another defect is that the control function remains inoperable even after the mat temperature falls to a level where the temperature control should run again. Also, it is difficult to identify the cause of operation failure, i.e., whether it is due to an abrupt increase of ambient temperature or to the malfunction of the sensor or the temperature controller.